org-latex: allow `org-export-latex-href-format' to have only one "%s"

This is useful when you want to use \url{link} instead of the default
\href{link}{path}.
Thanks to Henri-Paul Indiogine for bringing this up.
* org-latex.el (org-export-latex-href-format): docstring fix
to reflect the fact that you can use only one "%s".
(org-export-latex-links): allow `org-export-latex-href-format'
to have only one "%s".